Characterization of prototype BTeV silicon pixel sensors

before and after irradiation

Maria Rita ColucciaSimon Kwan

Fermi National Accelerator Laboratory

2001 IEEE Nuclear Science Symposium, San Diego 3-10 Nov Friday, Nov 8, 2001

Maria R. Coluccia - Fermilab 2

Outline

• BTeV SINTEF pixel sensor prototypes• Proton Irradiation at IUCF• Characteristics before and after irradiation• Conclusions

Maria R. Coluccia - Fermilab 3

BTeV SINTEF silicon pixel sensor prototypes tested

• n+/n/p configuration that allows them to operate partially depleted• Sensor thickness: 270 um• Low resistivity material: 1.0-1.5 KOhmxcm• P-stop electrode isolation technique• Oxygenated and non-oxygenated wafers• Various guard ring configurations

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P-stop sensorCommon p-stop Individual p-stop

n implantp implant gap between n and pgap between adjacent p

p implant n implantgap between n and p

bump pad

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Summary of the implant widths and gaps

We tested two different pixel arrays:Test cell sensors (12x92 cells) and FPIX1 sensors (18x160 cells)

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P-side guard ring designsThree different designs.

For the test cell sensors: 10 guard rings 18 guard rings

For the FPIX1 sensors: 11 guard rings

active area

10 GR 11-18 GR

metal

p-implant

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I-V curves before irradiation for standard SINTEF test cell

sensors

• 7 wafers tested • A few sensors had bad performance (high leakage current, low breakdown voltage) but this doesn’t depend on the p-stop layout

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I-V curves before irradiation for oxygenated SINTEF test cell

sensors

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I-V curves before irradiation for standard SINTEF FPIX1 sensors

For all these sensors we have a Vbreak of 300 V.This is due to the different p-implant width:1. For FPIX1_SIP (single individual p-stop) the gap between 2

adjacent p-stop rings is 3 um compared to 5 um in the test cell sensors

2. For FPIX1_SCP (single common p-stop) the p-implant width is 3um compared to 9um in the test cell sensors

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Breakdown Voltage Distribution

•Very high values (700 V) without significant differences between individual and common p-stop sensors and between oxygenated and standard sensors•Very high yield for the SINTEF wafers

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Irradiation test at IUCF (Indiana University Cyclotron Facility) with a 200 MeV proton beam

• 2 test cell standard sensors (individual and common p-stop) with 8 x 1013 p/cm2 • 4 FPIX1 sensors with 2 x 1014 p/cm2 : 2 oxygenated (individual and common p-stop) and 2 standard (individual and common p-stop)• 4 test cells sensors with 4 x 1014 p/cm2: 2 oxygenated (individual and common p-stop) and 2 standard (individual and common p-stop)Irradiation was done in air at room temperature.After irradiation the tested devices have been kept at –15 oC

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Leakage current: temperature dependence

After irradiation Ileak significantly increases, but the problem associated with the large current can be minimized by operating at reduced temperature.

)2

(2 Tk

E

leakB

g

eTI

Fluence: 8 x 1013 p/cm2

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Leakage current: fluence dependence

cmACT

/107.2 17230

eqTVI

)(

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I-V curves after irradiation with various fluences

We see no breakdown Voltage below 500 V for the test cell sensors.

standard sensors oxygenated sensors

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Capacitance: temperature and frequency dependence after

irradiation

A logarithmic change in frequency gives the same pattern of C-V’s as a linear change in temperature.

23 oC 40 KHz

Individual p-stop sensor (10 GR) fluence: 4 x 1014 p/cm2

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Depletion VoltageNo difference between oxygenated and standard sensors observed!

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Guard Rings: Voltage Distribution Before and After

Irradiation

Innermost guard ring

FPIX1_SCP oxygenated (11 GR)before after

Measurements performed with the innermost guard ring floating.We have a potential drop across the device edges after type inversion.

Innermost guard ring

29 V

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Conclusions•Excellent results for the SINTEF sensors•Very high yield•No significant difference between common and individual p-stop layout•Important effects introduced by different p- implant widths•No difference between oxygenated and standard sensors before and after irradiation for SINTEF sensors•More investigations needed for the guard ring structures•Next step: to study performance of the sensors bonded to ROC and charge collection efficiency before and after irradiation

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SINTEF wafer layout

We tested two different pixel arrays:1)Test cell sensors (12x92 cells)2) FPIX1 sensors (18x160 cells)

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After DicingWe diced several wafer:

• Some sensors present different result after dicing (high Ileak, low Vbreak)• Cleaning carefully the surface and the edges of the sensors we can restore the performances that we had before • All the sensors with three guard rings present performances degradation after dicing